Update BC guide with adding an exception #98
Description
The following documents should be updated to reflect that adding (throwing a new) exception is a breaking change:
- https://devdocs.magento.com/guides/v2.3/extension-dev-guide/versioning/dependencies.html
- https://devdocs.magento.com/guides/v2.3/extension-dev-guide/versioning/codebase-changes.html
- https://devdocs.magento.com/guides/v2.3/contributor-guide/backward-compatible-development/index.html
- this one covers modification of an exception type, but it's not a precise explanation. This should be improved to reflect a case when there was no exception and then it appears